Field of the Invention
The present invention relates to an image forming system of a tandem type serially connecting a plurality of image forming apparatuses, an intermediate conveying apparatus disposed between the image forming apparatuses, and an adjusting method of an amount of heat radiation from a sheet.
Description of the Related Art
There is known an image forming system of a tandem type configured by serially connecting a plurality of (e.g., two) image forming apparatuses (hereinafter, simply called “image forming system”). According to the image forming system, when images are formed on both sides of a sheet, an image can be formed on a front side of the sheet by an image forming apparatus on the upstream side, and an image can be formed on a rear side of the sheet by an image forming apparatus on the downstream side, for example. By causing both of the image forming apparatuses to share the processing of forming the images on the front side and the rear side of the sheet, it is possible to improve productivity compared to a case of forming the images on both sides of the sheet by one image forming apparatus.
Further, according to the image forming system, when different images are formed continuously on the same side of a sheet, it is also possible to form an image by the image forming apparatus on the upstream side and to form another image on the same face of the sheet by the image forming apparatus on the downstream side. By causing both of the image forming apparatuses to share the processing of forming the different images continuously on the same side of the sheet, it is possible also to improve the productivity compared to a case of forming the different images continuously on the same side of the sheet by one image forming apparatus.
In the image forming system, an intermediate conveying apparatus may be disposed between the image forming apparatus on the upstream side and the image forming apparatus on the downstream side, for stably conveying a sheet to the image forming apparatus on the downstream side and for reversing the front and rear sides of a sheet. FIG. 6 shows sheet length change when images are formed continuously on both sides of a plurality of sheets in such an image forming system, and FIG. 6A shows sheet length change at beginning of image formation, and FIG. 6B shows sheet length change after the number of sheets is increased.
The causes of the sheet length change may include thermal contraction of a sheet S having an image (solid-line character “A”) formed on the first side of one of the front side and the rear side by the image forming apparatus on the upstream side when the sheet S has passed through a fixing portion within the image forming apparatus on the upstream side, as shown in FIG. 6A. The sheet S is conveyed to the intermediate conveying apparatus in a state that the length (size in the lateral direction of FIG. 6) is shortened due to the thermal contraction.
After that, when the sheet S passes through a sheet conveying member within the intermediate conveying apparatus, a sheet conveying member robs the heat of the sheet S to reduce the temperature of the sheet S, thereby the length is restored by an amount corresponding to the temperature reduction, and then the sheet S is conveyed to the image forming apparatus on the downstream side.
In the image forming apparatus on the downstream side, the size and the position of an image (broken-line character “A”) to be formed on the second side which is remaining one of the front side and the rear side are adjusted according to the length of the sheet S conveyed from the intermediate conveying apparatus. Thereby, when the length of the sheet S returns to the original length after the sheet S has passed through a fixing portion of the image forming apparatus on the downstream side to complete image formation, the size and the position of the image formed on the second side come to match those of the image formed on the first side.
When images are formed continuously on a plurality of sheets, however, inside temperature of the intermediate conveying apparatus is raised by heat radiation from the sheet, and an amount of heat to be robbed from the sheet by the sheet conveying member is reduced as the number of sheets is increased. Accordingly, since the temperature reduction amount of the sheet is reduced when the sheet has passed through the intermediate conveying apparatus, as shown in FIG. 6B, the length of the sheet S conveyed from the intermediate conveying apparatus to the image forming apparatus on the downstream side becomes shorter than the length at the beginning of image formation.
As a result, if the size and the position of the image formed on the second side in the image forming apparatus on the downstream side are the same as those in initial adjustment, the size and the position of the image formed on the second side will not match those of the image formed on the first side.
FIG. 7 shows an example of a relationship between an amount of heat radiation from the sheet in the sheet conveying member within the intermediate conveying apparatus and a shift amount between the images on the front side and rear side of the sheet, in the case where images are continuously formed on both sides of a plurality of sheets. The amount of heat radiation from the sheet in the sheet conveying member is reduced and accordingly the shift amount between the image positions on the front side and the rear side of the sheet is increased, as the number of sheets is increased.
Conventionally, there has been proposed a technique of detecting the temperature of a sheet after an image has been formed on the front side, calculating a change amount of the sheet length according to the detection result, and changing a parameter related to an image to be formed on the rear side, as a technique of matching image sizes between the front side and the rear side when images are formed on both sides of the sheet (refer to patent literature 1, for example).
Further, conventionally, in an image forming system of a tandem type, there has been proposed a technique of detecting the temperature of a sheet passing through the intermediate conveying apparatus and cooling the sheet according to the detection result, as a technique of matching process conditions between the image forming apparatus on the upstream side and the image forming apparatus on the downstream side (refer to patent literature 2, for example).
Conventionally, there has been proposed a technique of preventing adhesion of sheets caused by toner melting, by detecting the temperature of the sheet conveying member which the sheet passes through, and cooling the sheet conveying member (refer to patent literature 3, for example).